Answer:
Try to look for the pattern.
As you can see on the first column, the last three numbers line up by +3 (12 to 15 to 18). Proof - 12 + 3 = 15 + 3 = 18.
So you go back by subtracting 3. 12 - 3 = 9 - 3 = 6 - 3 = 3
For the other column, the pattern is +17. Proof - 51 - 34 = 17, 102 - 85 = 17.
So do the same thing as in the first column.
Results:
3 17
6 34
9 51
12 68
15 85
18 102